Add action plugin for all_nodes data

This patch adds an action plugin to render the all_nodes group_vars for
the overcloud. This plugin will produce the same json structure as the
all_nodes.j2 template in the tripleo-heiradata role. Once
deploy_steps_playbook.yaml is migrated over to use the new action
plugin, the all_nodes.j2 template can be removed.
At scale, the template was taking an unusual amount of time to render.
Switching to native python with the action plugin results in improved
performance. In a 150 node environment, using the action plugin dropped
the render time from 7 minutes to 30 seconds.
Using an action plugin will also allow more easier unit testing (to be
forthcoming), and also add the opportunity to use debug statements to
show errors.
Change-Id: I92113589b98d7c1f9c05e1ea0938a03d95b7dd15
(cherry picked from commit 918452da7f)